1. Field of the Invention
The present invention relates to electrophotographic image-receiving sheets that have proper low-temperature toner fixability and excellent adhesion resistance and can provide high-gloss high-quality images, methods for producing the electrophotographic image-receiving sheets, and image forming methods using the electrophotographic image-receiving sheets.
2. Description of the Related Art
Electrophotographic processes are typically carried out under a dry condition with higher printing speed and may print on conventional papers such as regular papers and bond papers, therefore have been widely employed in copiers, printers of personal computers, etc. The electrophotographic image-receiving sheets, used in the electrophotographic processes, have at least a toner image-receiving layer on a support, and the toner image-receiving layer is produced by melting and extruding a thermoplastic resin composition on a support to form a layer, a coating liquid for a thermoplastic resin is coated on a support, for example. In recent years, a method for producing the toner image-receiving layer has been interested, in which a water-insoluble resin is employed in a form of aqueous dispersion of a thermoplastic polymer resin in view of minimum environmental load.
The thermoplastic resin of the toner image-receiving layer is typically an amorphous polymer that has a glass transition temperature Tg that is higher than the ambient temperature and lower by several tens degrees than the toner fixing temperature. Such an amorphous polymer may provide excellent adhesive properties with toner, but tends to suffer from adhesive problems such as coagulation of toner image-receiving layers while reserving and/or transporting in overlapped conditions due to higher adhesive force between toner image-receiving layers.
On the other hand, crystalline polymers have low adhesive forces at normal temperature even the glass transition temperature Tg is lower than 0° C. thus are free from adhesive problems between toner image-receiving layers, meanwhile tend to melt rapidly above their melting temperatures specific for the resins. As such, the crystalline polymers have potential features in terms of excellent preserving and fixing properties, which have been tried to apply to the electrophotographic image-receiving sheets.
Japanese Patent Application Laid-Open (JP-A) No. 2005-92097 discloses that a color electrophotographic sheet, of which the toner image-receiving layer being formed of a certain crystalline polyester, may embed toner images uniformly into the toner image-receiving layer at a fixing temperature lower than previous one, bring about high-quality images with smaller unevenness from paper surface, and also afford proper mechanical durability with respect to folding and/or bending at processing stages.
JP-A No. 2005-99123 discloses that an image support, having a light diffusion layer and a toner receiving layer on a base material in which the toner receiving layer being formed from a polyester resin of melted and mixed amorphous and crystalline polyester resins, may improve the mechanical strength and heat resistance and enhance the low temperature fixability.
However, the toner receiving layer is produced in a melting and extruding process, which leading to expensive production systems. Moreover, it is likely that the production process is energy-consuming, the production cost is expensive, and environmental load is significant. In addition, there is such a problem that the crystalline polymer tends to loss its crystallinity while the crystalline polymer and the amorphous polymer is heated, melted and mixed to form a film, which possibly leading to poor performance and insufficient gloss for photographic images depending on conditions and/or combinations.
JP-A Nos. 2005-181881 and 2005-181883 disclose that an electrophotographic sheet, of which the toner image-receiving layer contains an amorphous polymer and a crystalline polymer, may improve the adhesion resistance that is a defect for amorphous polymers as well as the adhesive properties with toner resins that are a defect for crystalline polymers, thus proper toner fixability and excellent adhesion resistance may be combined together with, and images may be formed with high gloss and high quality.
However, the amorphous polymer and the crystalline polymer are dissolved in an organic solvent, in which the both can dissolve, to prepare a coating liquid which being then coated and dried, thus suffering from a significant environmental load. In addition, there is such a problem that the crystalline polymer tends to loss its crystallinity while being coated and dried as a mixture, which possibly leading to poor performance depending on conditions and/or combinations. Moreover, high gloss images may be formed under higher fixing temperatures, however, the gloss tends to decrease and/or undesirable defects like nonuniform gloss tend to generate at border lines between images and non-image areas at lower fixing temperatures, thus it is difficult to form appropriate images.
As described above, prior literatures describe no more than melting and extruding processes or coating processes with organic solvents that are undesirable due to a significant environmental load, and no electrophotographic image-receiving sheets have been investigated in combination with aqueous dispersions of crystalline polymers. This is derived from that conventional crystalline polymers are hardly soluble in usual organic solvents and it is difficult to prepare an aqueous substance and/or a stable dispersion. The preparation of aqueous substance has been investigated as regards very limited crystalline polymers that are unsatisfactory for electrophotographic image-receiving sheets in view of their properties; that is, crystalline-polymer aqueous dispersions have not been applied substantially at all to electrophotographic image-receiving sheets heretofore.